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BIG PHYSICS, BIG QUESTIONS –

Flies are spreading antibiotic resistance from farms to people

To blame for resistance in summer?

Digital Vision/Getty

By Debora MacKenzie

It is now the year of the chicken in China – in more ways than we knew. The first systematic study of bacterial resistance to last-resort antibiotics on farms and hospitals in China has revealed far more resistance than standard tests had previously suggested, especially on chicken farms and meat. Worse, the study reveals for the first time that the genes that give bacteria their resistance are being spread by flies.

When carbapenems fail, one of the few options left is the antibiotic colistin, but in 2015, colistin resistance was discovered in China. The genes for both types of resistance can spread between different types of bacteria.

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The colistin resistance gene, mrc-1, has now been found in 25 countries, on four continents. It was first detected in China, though it is not known if it evolved there. It could well have, however: unlike in western countries, in China colistin is not used as an antibiotic in people, but 8000 tonnes of the drug is given to animals as a growth promoter every year, mainly to pigs and chickens.

In April, this practice will be banned in China, and colistin will begin to be used to treat people instead. But it may be too late.

Carried by flies

In a systematic search for colistin and carbapenem resistance in several regions of China, Tim Walsh at Cardiff University, UK and colleagues found colistin resistance in around one per cent of hospital patients in two large cities – even though the drug has not been used to treat people there. Their results were published last week (The Lancet, 10.1016/S1473-3099(16)30527-8).

Now we know that the resistance genes probably came from a farm. In a related study, published today, the same team reports that a third of the Escherichia coli bacteria sampled from chicken farms and meat in grocery stores resisted carbapenems, and a quarter of those also resisted colistin.

What’s more, the genes have wings. The team found high rates of bacteria with colistin and carbapenem resistance genes in dog faeces from chicken farms, and in the flies at these farms. This is the first time such a result has been reported, and suggests that flies could be spreading resistance from farm animals.

“Their ability to contaminate the environment has immense public health concerns,” the team concludes. It may be why hospital patients who lived far away from farms were not less likely to have a resistant infection during summer, says Walsh. “In the summer flies will carry those bacteria everywhere.”

Spread by swallows

Unexpectedly, when the team sequenced the entire genomes of the bacteria, far more turned out to be silently carrying those resistance genes than actively using them. Nearly all the bacteria sampled on chicken farms had mrc-1, though only half resisted colistin. This means the potential for antibiotic resistance is likely vastly underestimated by standard tests.

The team concluded that the DNA sequences of bacteria from chicken farms, slaughterhouses, supermarkets and people were so similar that colistin and carbapenem resistance must have spread first in the poultry sector and then to people. It’s compelling evidence, to add to previous studies, that antibiotic resistance in agriculture affects people, says Lance Price at George Washington University, Washington D.C., who has found resistant bacteria on supermarket meat in the US.

“It worries me that Chinese officials are going to start using colistin in human medicine,” says Price, saying that this could cause an explosion of human infections that are already silently carrying mcr-1 from chickens.

The problem could spread. Walsh’s team also found resistant bacteria in faeces from swallows on farms in China. These birds will likely carry this resistance with them as they migrate to southeast Asia. Walsh fears that, when antibiotic manufacturers can no longer sell tonnes of colistin to farmers in China, they will export it countries like Vietnam and Thailand, laying the foundations for an explosion in resistance there too.